The ultimate membrane would have identical, highly interconnected pores comprising a porespace with perfect three-dimensional periodic order. This ideal has been approached in the development of polymeric microporous membranes but never achieved. The simplest type of sieve is a net filter, where each layer in the filter is a woven mesh. The geometry of the pore space in a given layer is thus a close approximation to a finite portion of a doubly-periodic net, the latter being a mathematical idealization with perfect regularity within the plane. Note that if, in addition, these doubly-periodic layers are stacked at regular intervals with all layers in vertical registry, the resulting sieve is triply-periodic. Woven mesh filters are not available with pore sizes less than about 60 microns, so they cannot be used for reverse osmosis, ultrafiltration, nor even microfiltration.
Another doubly-periodic geometry that is achieved in some filters is that of hexagonally close-packed cylindrical pores. For example, glass capillary bundle filters are made from close-packed arrays of parallel glass capillaries. Capillary arrays can also be formed from hollow fibres of organic polymers, although these are not yet available commercially. A major drawback of cylindrical-pore filters is the lack of porespace branchings and reconnections, which leaves only one pathway for a fluid particle entering a given pore; thus clogging becomes a serious problem, as does sensitivity to handling. Of course, cylindrical pores can provide a narrow distribution of pore sizes without necessarily lying on a doubly-periodic lattice; for example, nucleation-track filters have randomly placed parallel cylindrical pores. But this randomness means that the number of pores per unit cross-sectional area must be kept small to maintain monodispersity, so that these filters have the additional drawback of low porosity and thus low filtration rates. Nevertheless, nucleation-track filters are considered the best membrane filters available for sieving below 60 microns, despite these obvious drawbacks.
U.S. Pat. No. 4,280,909 describes a microporous membrane which is, strictly speaking, triply-periodic, but the topology of the porespace is exactly the same as in the capillary array membranes, namely the flow channels are strictly linear and there are no porespace branchings or reconnections. The periodicity in the third dimension refers only to the vertical stacking of tapered pores of equal height, so that the cylindrical pores of the capillary array membrane have become instead tubular pores with a periodically varying diameter. This membrane does not satisfy one of the most important desired features, namely the intricate yet controlled porespace. A precisely defined porespace with branching and reconnections, in which each identical pore body connects to exactly the same number of other pore bodies through identical pore throats, is important in:
a) reducing clogging, as when the membrane is used for filtration, for example; PA1 b) enhancing mixing, as when the membrane is used in catalysis or ion exchange, for example; and, PA1 c) providing accessible channels and pore bodies of specific shape, as when the membrane is used in the preparation of metal microstructures [Jacobs et al. 1982], for example.
Sintered-particle membranes have intricate three-dimensional porespaces with many interconnections, but have oddly-shaped and polydisperse pores as well as low pore density, the latter drawback being the primary reason they have been generally replaced by membrane filters. Most sintered-particle filters have retention ratings at or above 0.7 microns.
The membrane that is most commonly used in particle filtration has high porosity but a random, irregular porespace that makes it generally unusable as a sieve. Distributions of pore radii in cellulose nitrate membrane filters have been measured using mercury porisimetry, and the distributions are very broad: the full-width at half-maximum (FWHM) of the distribution is about equal to the average radius [Brock 1983].
In the realm of nonpolymeric sieves, zeolites provide fairly well-controlled, triply-periodic pore networks, but the free diameters of aperatures governing access to channels are generally less than 2 nm, and in fact nearly always less than 1 nm [Barrer 1978]; also the porosities of zeolites (defined as cc's of water per cc of crystal) are nearly always less than 50% Furthermore, most zeolites selectively absorb polar molecules because most are themselves highly polar, having high local electrostatic fields and field gradients [Barrer 1978]. Perhaps most importantly, the macroscopic size of zeolite crystals has very serious practical limitations making such materials unsuitable for forming reasonably large membrane-like structures with the necessary degree of continuity.
These and other difficulties with prior materials and methods have been obviated in a novel and inventive manner by the present invention.